1. Field of the Invention
This invention relates to television cameras, and more particularly to black shading correction for television cameras.
2. Background of the Invention
Television cameras are in widespread use in a variety of sensing, communication, and scientific applications. With the advent of improvements such as high-definition systems, camera defects will become more apparent to the television viewer. Thus, camera improvements are desirable. One of the defects to which cameras are subject is "dark current" or "black shading", which is current or charge (signal) which arises from the characteristics of the camera imager (the actual photosensitive screen) itself, and which does not depend on the image falling thereon.
The dark signal may be viewed as being the imager signal when a cap is placed on the associated lens to eliminate light from the image. In the context of charge-coupled device (CCD) imagers, the dark current may be viewed as "leakage" which results in charge which accumulates in each picture element (pixel) of the "A" register of the imager during the image integrating interval. Thus, each pixel includes a charge portion attributable to the integration of image information during the integrating interval, and also includes a charge portion attributable to dark current over the same interval.
In general, the pixels of a CCD imager are similar to the other pixels, except in the case of point defects, so the dark signal contribution in each pixel tends to be the same as in the other pixels. Thus, it might be thought that subtraction of a single value of charge from each pixel might be sufficient to correct for the presence of dark current. However, charge is read from the pixels of the A register of a CCD imager by moving columns of pixel information simultaneously along the A register into a "B" register, where the information is stored for further processing. During the transfer of charge from the A register to the B register, dark current continues to accumulate in the pixels, with the result that those pixels which dwell in the A register for the longest period of time tend to accumulate more charge from the dark current than those which dwell for a lesser time. Thus, the "upper" pixels, which must traverse the entire A register during the charge transfer "pull-down" period before arriving at the B storage register, accumulate more charge from the dark current than those at the bottom of the A register, which arrive at the B register earlier.
This effect causes a "shading" across the imager, which in principle requires a ramp-like correction signal to be subtracted from the imager signal, with the ramp waveform applied in each column of read-out image information, from bottom to top of the image, for correction thereof. A parabolic correction waveform is sometimes necessary. A further correction waveform may be necessary along each row, in which case the correction waveforms are summed to produce the total correction waveform to be subtracted from the imager signal.
In the case of vacuum-type imaging tubes, the photosensitive screen is subject to dark signal variations attributable to such conditions as slight thickness variations of the photosensitive material across the screen, variations in its conductivity, or in the interface between the material and the underlying substrate. Thus, the dark signal correction required in a tube-type imager may be more complex than that required in a CCD imager.